1. Field of the Invention
The present invention relates to a belt device and an image forming apparatus.
2. Description of the Related Art
In an image forming apparatus of an intermediate transfer system among various image forming apparatuses, such as copiers, printers, facsimile machines, and multifunction peripherals having functions of a copier, a printer, and a facsimile machine, a toner image formed on a photosensitive drum is transferred onto a transfer belt (primary transfer) and then the toner image on the transfer belt is transferred onto a recording sheet (secondary transfer). The belt device having a transfer belt of this type includes a cleaning unit on the upstream side of the photosensitive drum in order to remove non-transferred toner attached to the transfer belt. An example of the belt device having a cleaning unit of this type is disclosed in, for example, Japanese Patent Laid-open Publication No. 2008-9011 (hereinafter, described as “Patent Document 1”).
In the belt device disclosed in Patent Document 1, a photosensitive drum on the most upstream side is disposed such that the center shaft thereof is positioned closer to the transfer belt than the center shafts of other photosensitive drums so that the amount of contact between the transfer belt and a cleaning opposing roller can be increased to improve the cleaning performance of the cleaning unit (see, for example, a paragraph [0036] of Patent Document 1).
However, in the configuration disclosed in Patent Document 1, the belt is wound around the opposing roller by being stretched outward by the opposing roller. Therefore, the opposing roller needs to be disposed at an offset position outward from a belt extended line between the photosensitive drum located on the most upstream side and the driving roller. Therefore, the thickness of the belt device (the maximum size between the outer periphery on a belt forwarding side and the outer periphery on a belt returning side) increases, resulting in an increase in a space needed for the belt device in the thickness direction. In particular, when the angle of contact between the belt and the opposing roller is increased to improve the cleaning performance, the opposing roller needs to be disposed further outward, and therefore, the above-mentioned problems become more apparent.
Meanwhile, when the image forming apparatus switches from a color mode to a monochrome (single color) mode, there is a demand to stop and separate the photosensitive drums other than the photosensitive drum used in the monochrome mode from the transfer belt in order to reduce abrasion of the surfaces of the photosensitive drums or to save power consumption. Therefore, there is a need for the belt device that can meet the above demand.
Therefore, there is a need to provide a belt device and an image forming apparatus that can easily cope with image formation in the monochrome mode and that are compact in size.